This patent application claims priority based on a Japanese patent applications H11-062636 filed on Mar. 10, 1999 and H11-065948 filed on Mar. 12, 1999 the contents of which are incorporated herein by reference.
The present invention relates to a sintering apparatus which dehydrates and sinters a porous glass base material serving as a raw material for an optical fiber so as to produce a transparent glass, and a method of sintering the porous glass base material. 2. Description of the Related Art
An optical fiber is made of the porous glass base material, as a raw material, comprised of: a core in which silicon tetrachloride (SiCl4) as a main material and a doping material having increasing refractive index are vaporized and are hydrolyzed in the oxyhydrogen blaze so that the glass particulates thus produced are deposited and which presents a high refractive index; and a clad in which glass particulates are deposited and whose reflection rate is lower than that of the core. The porous glass base material is usually produced such that the glass particulates are deposited by a vapor-phase axial deposition (VAD) method, outside vapor deposition (OVD) method or the like. In a furnace tube into which the porous glass base material is inserted, the base material is heated and sintered so as to dehydrate and perform the transparency-vitrification process on the base material in the dehydration-reactive gas and inert gas atmosphere while the base material descends, so that an optical fiber preform comprised of a core and a clad can be obtained. After this optical fiber preform is elongated at a desired diameter and is then drawing-performed by a drawing apparatus, an optical fiber can be obtained.
During the sintering of this porous glass base material, conditions such as the flow rate of the dehydration-reactive gas and inert gas, the passing (descending) speed of the base material through a heating region, and the heating temperature in the heating region are kept constant. With each of such conditions being kept constant, the thermal energy necessary for processing the dehydration and the transparency-vitrification is supplied evenly to the porous glass base.
However, the heat thus supplied is not sufficient in order to perform the sintering uniformly. As a result, the sintered portion and unsintered portion do not become concentric, and those are mixed in an asymmetric manner. Moreover, when the unsintered portion is sintered, the glass which was already sintered and thus had low viscosity is also involved unwantedly so as to contract unevenly. Thus, the core is displaced from the axial center of the base material so as to cause eccentricity. This eccentricity further causes the supply of the thermal energy to become non-uniform, so that the periphery of the base material tends to present the form of an ellipse. Let us call this tendency ellipsization. Once the eccentricity and ellipsization occur, the sintering process proceeds while the non-uniform contraction occurs continuously. Therefore, the whole of the optical fiber preforms suffer from the eccentricity and ellipsization.
Consequently, the optical fiber obtained by performing the drawing process on these optical fiber preforms present the eccentricity and ellipsization. When these optical fibers thus produced are connected to each other by means of adhesion or fusion, contact loss is caused because of difference of shapes in coupling surfaces between the core and clad, displacement of the axial center of the core, and the difference in core diameters.
When manufacturing the porous glass base material, there is available a method by which the clad will be further grown around its peripheral by the OVD method after the core and clad are formed by the VAD method. Now, in the conventional practice, the sintering is carried out in a manner that the porous glass base material is sintered while keeping constant the flow rate of the chlorine gas and inert gas serving as the dehydration-reactive gas as well as keeping the descending speed of the base material constant and keeping the condition of the heating temperature fixed. However, there occurs a difference between remaining rates of the dehydration-reactive gases in the clad formed by the VAD method and the clad formed by OVD method. If the 1000 ppm chlorine gas remains in the optical fiber sintered, the refractive index increases by approximately 10xe2x88x925. Thereby, the difference between the chlorine gas remaining rates in the clad causes a great deal of refractive index difference in the clad. Thus, the distribution characteristics of the optical fiber material are deteriorated. Moreover, as the porous glass base material descends in the furnace tube during a sintering process, a temperature gradient in the furnace tube and the flow rate of gas within the furnace tube change, so that the remaining rate of the chlorine gas in the axial direction of the base material becomes non-uniform and the difference in the refractive indexes fluctuates, thus causing a problem where the cut-off wavelength can not be kept constant in the axial direction.
Therefore, it is an object of the present invention to provide sintering apparatus for porous glass base material, as well as a method therefor, which does not cause the eccentricity of the core and the ellipsization of the periphery of the glass material while sintering and dehydrating the porous glass base material and performing the transparency-vitrification process on the porous glass base material. It is another object to provide sintering apparatus, as well as a method therefor, capable of suppressing the refractive indexes inside the clad in the event that the porous glass base material is sintered and dehydrated and transparency-vitrification processed. These objects are achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
According to one aspect of the present invention, there is provided apparatus for sintering a glass base material which is a base material for an optical fiber, the sintering apparatus comprising: a control unit which varies a condition for sintering the glass base material; and a furnace which sinters the glass base material by heating the glass base material in an atmosphere of dehydration gas and inert gas, the control unit including a drive source which supplies the glass base material to the furnace.
Preferably, the control unit includes a first sensor, connected to said drive source, which detects a position of the glass base material inside said furnace so that the drive source varies the speed of the glass base material which descends through the furnace, based on the position of the glass base material, detected by the first sensor.
The furnace further includes: a heating source, provided in the periphery of said furnace, which heats the glass base material; and a gas introducing pipe, connected to said furnace, which introduces a chlorine gas serving as the dehydration gas, and a helium gas serving as the inert gas into said furnace.
Preferably, the drive source may vary the speed at which the glass base material descends in a manner that the descending speed of the first half of the glass base material in a region corresponding to said heating source is slower than that of the end half of the glass base material in said region.
Moreover, a maker may be provided in the glass base material so that the position of the glass base material can be detected by the first sensor.
Moreover, the descending speed of the base material may be changed at once or varied in a gradual manner or changed in a step-by-step manner.
According to the another aspect of the present invention, there is provided a sintering apparatus, wherein the control unit includes a temperature control unit which controls the temperature of said heating source via a second sensor provided in said furnace.
The temperature control unit may perform a proportional integral and differential operation.
Moreover, the gas introducing pipe may branch out to a dehydration-reactive gas passage for introducing the chlorine gas and an inert gas passage for introducing the helium gas.
Moreover, there may be provided a flow rate control unit which varies a flow rate ratio of dehydration-reactive gas to that of the inert gas.
Moreover, the dehydration-reactive gas passage may include a first flow rate meter and a first flow rate control valve, and the inert gas passage may include a second flow rate meter and a second flow rate control valve, and there may be provided a flow rate control unit connected to both the first and second flow rate meters and there may be provided a second drive source connected to the flow rate control unit and the first and second flow rate control valves.
Moreover, it is preferable that the ratio of gas passing through the dehydration-reactive gas passage to gas passing through said inert gas passage is in the range of approximately 10% to 50%.
According to still another aspect of the present invention, there is provided apparatus for manufacturing an optical fiber, comprising: a glass base material generating apparatus which generates glass base material serving as a raw material for the optical fiber; a glass base material dehydrating and sintering apparatus which dehydrates and sinters the glass base material, the glass base dehydrating and sintering apparatus including: a control unit which varies a condition for sintering the glass base material; and a furnace which sinters the glass base material by heating the glass base material in an atmosphere of dehydration gas and inert gas, wherein the control unit includes a drive source which supplies the glass base material to the furnace; a glass base material first elongating apparatus which elongates the glass base material to generate a glass rod; a glass rod second elongating apparatus which elongates the glass rod for the second time to generate a preform which is parent material of the optical fiber; and a preform drawing apparatus which draws the preform to produce the optical fiber.
According to still another aspect of the present invention, there is provided a method of sintering a glass base material which is a base material for an optical fiber, the sintering method comprising: varying a condition for sintering the glass base material; the varying condition including: supplying the glass base material to a furnace; and sintering the glass base material in the furnace by heating the glass base material in an atmosphere of dehydration gas and inert gas.
According to still another aspect of the present invention, there is provided a method of manufacturing an optical fiber, comprising: generating a glass base material serving as a raw material for the optical fiber; varying a condition for sintering the glass base material; the varying condition including: supplying the glass base material to a furnace; and sintering the glass base material in the furnace by heating the glass base material in an atmosphere of dehydration gas and inert gas; elongating the glass base material to generate a glass rod; elongating the glass rod for the second time to generate a preform which is a parent material for the optical fiber; and drawing the preform to produce the optical fiber.
This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.